1. Field of the Invention
The present invention relates to a novel pressure sensitive adhesive composition which has excellent blister resistance, excellent removability and no corrosive property against substrate and novel pressure sensitive adhesive sheet, label and laminate utilizing the adhesive composition and useful for attaching the sheet, the label and the laminate to molded articles. More particularly, the present invention relates to a novel pressure sensitive adhesive composition which has excellent cohesive strength and heat resistance and is favorably utilized for surface protecting sheets having resistance against formation of blisters, such as bubbles and lifting, on application to the surface of a substrate and good removability without leaving dirty remainings and for polyester tape electric insulators having no corrosive property against metals, such as copper wires, and also relates to novel pressure sensitive adhesive sheet, label and laminate having excellent resistance against formation of blisters by utilizing the adhesive composition.
2. Description of the Prior Art
As is well known, pressure sensitive adhesive sheets having a coating layer of a pressure sensitive adhesive are widely used. This is because the pressure sensitive adhesive sheets can be readily bonded to the surface of various substrates conveniently by merely putting on and lightly pressing to the substrate surface and, different from adhesives in general, the pressure sensitive adhesive forming the coating layer of the sheet contains no organic solvent so that no problem of environmental pollution are caused thereby.
As a consequence of the trend in recent years that light weight articles are preferred, the quantity of plastic products is rapidly increasing year by year. Thus, the occasion in which pressure sensitive adhesive sheets are attached to plastic goods or plastic packages of goods as a label is also increasing rapidly. Pressure sensitive adhesive sheets are quite often applied to the surface of polycarbonate resin plates for the purpose of protection of the surface from scratching and adjustment of sun light transmission.
Pressure sensitive adhesive sheets in general have a serious problem that, when a pressure sensitive adhesive sheet is applied and bonded to the surface of an article made of acrylic resin, polycarbonate resin and the like, so-called blisters are not rarely formed due to incomplete squeeze out of the gas between the substrate surface and the pressure sensitive adhesive sheet attached as a label or a protecting sheet. This problem has been unavoidable when the face stock film of the pressure sensitive adhesive sheet is made of a resin having gas barrier property such as conventional polyester resins. This problem is particularly serious at high temperatures or in conditions of high temperature and high humidity.
When blisters are caused in the pressure sensitive adhesive sheet attached to the surface of an article as a label, not only the article has a very unsightly appearance to decrease the commercial value but also the label and the like attached to the surface are liable to eventual falling. In a printed label made of a see-through pressure sensitive adhesive sheet, in particular, it is unavoidable that the appearance is extremely poor because the layer of the adhesive coating exhibits a pockmarked or white and cloudy appearance through the face-stock film.
Surface protecting sheets made from pressure sensitive adhesive materials coated on the surface of paper or synthetic resin films are widely utilized for metal plates, such as stainless steel plate, aluminum plate, steel plates and the like, glass products, molded resin products, such as ABS resin plates, acrylic resin plates and styrene resin plates, and the like other various products for the purpose of preventing corrosion and staining of the surface.
Pressure sensitive adhesive sheets prepared by coating a pressure sensitive adhesive material comprising as the main component thereof natural rubber or synthetic rubber compounded with resinous materials on the surface of a substrate, such as paper and synthetic resin film, have been widely used for the surface protecting sheets. In the pressure sensitive adhesive material used for these pressure sensitive adhesive sheets, though the low initial adhesive strength required for attaining good removability can be provided, adhesive strength is increased during storage time remarkably. The increase of the adhesive strength causes a problem that removal of the pressure sensitive adhesive sheet from the surface of the substrate becomes very difficult and, moreover, a part of the adhesive material very often remains on the surface of the substrate because of inferior stability of the pressure sensitive adhesive material against weathering.
Recently, acrylic pressure sensitive adhesive materials are widely used for the surface protecting sheets because of the good resistance against weathering. When the acrylic pressure sensitive adhesive material is utilized for the surface protecting sheets, the adhesive material is required to have high internal cohesive strength so that it has good removability and does not have adhesive transfer to the surface of the substrate. By this reason, acrylic polymers utilized in the adhesive material for the surface protecting sheets are modified by copolymerization with a monomer having carboxyl group as a functional group and the copolymer prepared are compound with a polyisocyanate compound, melamine resin, metal salts and the like as the external crosslinking agent and then crosslinked.
However, the acrylic pressure sensitive adhesive material has a problem that the adhesive material must be aged by standing for a long time at the room temperature or by heating because the adhesive material has low rate of crosslinking and the initial adhesive strength can not be kept at a low value immediately after the application to the substrate. The acrylic pressure sensitive adhesive material has another problem that adhesive strength is inevitably increased during storage time because the adhesive material contains copolymer comprising a monomer having a carboxyl group and removability from the substrate is poor.
In some cases, surface active agents, silicones, plasticizers or inorganic salts are added to the pressure sensitive adhesive material for the purpose of preventing the increase of the adhesive strength. However, this method has a problem that deficient printing and delamination sometimes occur because of contamination of the surface of substrate by bleeding of the added ingredients.
For the purpose of increasing rate of crosslinking and improving efficiency of processing, density of crosslinking in the pressure sensitive adhesive material is increased and cohesive strength is enhanced by using a copolymer comprising two or more kinds of monomer having functional groups, such as carboxyl group, methylol group, glycidyl group and the like, followed by crosslinking with an external crosslinking agent, such as polyisocyanate compounds, melamine resins, metal salts and the like. However, this type of pressure sensitive adhesive material is lacking in anchoring power to paper or synthetic film to cause problems such as transfer of the adhesive material to the substrate and removal or delamination during application to the substrate because of low adhesive strength caused by too high cohesive strength.
As exemplified above, conventional pressure sensitive adhesive sheets for surface protection do not satisfy the requirement that the non-staining property to the substrate and the balanced adhesive-removable properties are simultaneously attained.
Excellent adhesive strength, creep property and solvent resistance are required for polyester tapes for electric insulation as good industrial materials. Electric and electronic apparatus have been making progress in the direction of decreased size, light weight and higher performance in recent years and, along with this progress, tapes having excellent corrosion resistance, particularly resistance against electro-corrosion and resistance against coloring of copper, are being required more uregently. Acrylic pressure sensitive adhesive materials using comonomers having carboxyl group or hydroxyl group in a lowest possible amount have been mainly utilized for polyester tapes for electric insulation to achieve better balance between adhesive strength, creep properties and weatherability. However, pressure sensitive adhesive tapes having sufficiently excellent corrosion resistance have not been successfully prepared.